This invention relates to the preparation of polyurethane foams. This invention further relates to the use of certain gel catalyst compositions to prepare polyurethane foams exhibiting high levels of oxidative stability relative to products obtained using conventional organotin catalysts or combinations of these organotin catalysts with compounds containing phosphorus and halogen. Certain of these catalyst compositions are novel and constitute part of this invention.
Flexible polyurethane foams are widely used in the manufacture of furniture, particularly seat cushions, and as packaging material for delicate instruments and other articles that are susceptible to damage during handling and transit. If the foam is incorporated into a piece of furniture or other durable product, the foam must withstand exposure of several years or longer to elevated temperatures, atmospheric oxygen, or both without undergoing significant degradation, as evidenced by a gradual loss of resiliency and structural strength which may ultimately culminate in disintegration of the foam.
It is well known to prepare flexible cellular polyurethanes by reacting polyols containing two or more reactive hydrogen atoms, as determined by the Zerewitinoff method, with polyfunctional isocyanates in the presence of a polymerization or gel catalyst and a blowing agent such as water or a relatively low boiling fluorocarbon. A surfactant is often included in the reaction mixture together with a blowing catalyst to obtain the desired uniformly small cell size within the foam when water is used as the blowing agent. Fillers such as barytes and various additives such as flame retardants can also be present in the formulation employed to prepare the foam.
U.S. Pat. No. 3,620,985 discloses that organotin compounds are effective gel catalysts for cellular polyurethanes. Compounds of the formula R.sub.a SnX.sub.4-a wherein R is typically butyl or other alkyl radical containing from 1 to 20 carbon atoms and X is a halogen or other anionic radical are generally less than satisfactory for preparing flexible foams that are exposed to atmospheric oxygen, elevated temperature, i.e. above about 50.degree. C., or both over extended periods of time. Under these conditions the foams may lose resiliency and structural integrity to the extent that they disintegrate when compressed. This is particularly true of those foams derived from polyether-containing polyols that are prepared from propylene oxide. These polyols may also contain ester linkages, (--OOCRCOO--), and end groups derived from ethylene oxide to obtain hydroxyl groups bonded to primary rather than secondary carbon atoms. This susceptibility to degradation can be decreased by employing polyols that contain side chains obtained by the graft polymerization of acrylonitrile, styrene or other vinyl monomer onto poly(propylene oxide), however even these foams undergo a significant decrease in structural strength and become brittle following prolonged exposure to oxygen and/or heat when prepared using conventional organotin catalysts such as dibutyltin dilaurate.
It is known that the resistance of flexible polyurethane foams based on polyether-containing polyols to oxidatively and thermally induced degradation can be improved by employing compounds containing phosphorus and a halogen in combination with an organotin gel catalyst. Catalyst compositions of this type are disclosed in U.S. Pat. Nos. 3,067,149; 4,048,100; 4,052,346 and British Pat. No. 1,003,201. This type of catalyst composition may be suitable for certain applications where resiliency is not a requirement, since foams prepared using these catalyst compositions typically recover only a small fraction of their original height within 30 minutes after being compressed to 10% of their original height for 22 hours at a temperature of 70.degree. C., a conventional method for determining resiliency and dry heat stability of flexible polyurethane foams.
It is possible to improve the dry heat stability without adversely affecting the resiliency of flexible polyurethane foams prepared from polyether-containing polyols using gel catalyst compositions containing an organotin compound, phosphorus and a halogen. Surprisingly it has now been found that if the phosphorus and halogen are both present in the same or different compounds and each compound contains a tetravalent tin atom that is in turn, bonded to at least one hydrocarbyl group, the resultant foam exhibits improved thermal stability relative to foams prepared using an organotin compound as the gel catalyst in the absence of compounds containing phosphorus and a halogen. The resiliency exhibited by the foam following prolonged compression can be further improved by including a mono- or dicarboxylic acid in the catalyst composition.